Wave division multiplexing (WDM) increases bandwidth in optical communications by providing for communication over several wavelengths or channels. For long haul optical communications the optical signal must be periodically amplified. To maximize WDM capacity, it is desirable that the optical bandwidth of the system be as wide as possible. Raman amplification is one of the amplification schemes that can provide a broad and relatively flat gain profile over the wavelength range used in WDM optical communications. (See Y. Emori, “100 nm bandwidth flat-gain Raman Amplifiers pumped and gain-equalized by 12-wavelength channel WDM Diode Unit,” Electronic Lett., Vol. 35, no 16, p. 1355 (1999) and F. Koch et. al., “Broadband gain flattended Raman Amplifiers to extend to the third telecommunication window,” OFC'2000, Paper FF3, (2000)). Raman amplifiers may be either distributed or discrete (See High Sensitivity 1.3 μm Optically Pre-Amplified Receiver Using Raman Amplification,” Electronic Letters, vol. 32, no. 23, p. 2164 (1996)). The Raman gain material in distributed Raman amplifiers is the transmission optical fiber, while a special spooled gain fiber is typically used in discrete Raman amplifiers.
Raman amplifiers use stimulated Raman scattering to amplify a signal at a signal wavelength. In stimulated Raman scattering, radiation power from a pump radiation source is transferred to an optical signal to power from a pump radiation source is transferred to an optical signal to increase the power of the optical signal. The frequency (and therefore photon energy) of the radiation emitted by the pump radiation source is greater than the frequency of the radiation of the optical signal. This down shift in frequency from the pump frequency to the signal radiation frequency is due to the pump light interaction with optical phonons (vibrations) of the Raman gain material, i.e., the medium through which the pump radiation and the optical signal are traversing.
The Raman gain material in Raman amplifiers can be the transmission optical fiber itself. The Raman gain coefficient for a silica glass fiber (such as are typically used in optical communications) is shown in FIG. 1 as a function of the wavelength shift relative to a pump wavelength of around about 1400 nm. As can be seen, the largest gain occurs at about a 100 nm shift. Thus, the maximum gain for a single pump wavelength of about 1400 nm will occur at a signal wavelength of about 1500 nm. Since the optical gain is proportional to the pump intensity, the gain of the signal of a Raman amplifier is the product of the Raman gain coefficient and the pump intensity.
The gain profile having a typical bandwidth of 20-30 nm for a single pump wavelength is too narrow for WDM optical communications applications where a broad range of wavelengths must be amplified. To broaden the gain profile, Raman amplifiers employing multiple pump wavelengths over a broad wavelength range have been suggested for use in WDM optical communication applications. For example, it has been suggested to use twelve pump wavelengths to achieve a 100 nm bandwidth Raman amplifier.
In order for a flat gain profile to be achieved, the pump-pump interactions generally require that the shorter pump wavelengths have a higher pump power than the longer pump wavelengths. This is so because energy from the shorter wavelength (higher photon energy) pumps is transferred to the longer wavelength pumps due to stimulated Raman scattering. To compensate for the pump-pump energy loss at shorter wavelengths, the shorter pump wavelengths should have increased power.
A typical pump power-pump wavelength scheme to achieve a relatively flat and broad Raman gain profile is illustrated in FIG. 2 for the case of twelve pump wavelengths. As can be seen in FIG. 2, the pump power decreases for increasing wavelength. Also, the spacing between wavelengths is closer for shorter wavelengths. FIG. 3 illustrates a relatively flat and broad Raman gain profile for a pump power-pump wavelength scheme similar to that of FIG. 2. The variations on the gain spectrum result in channel-to-channel variation in the optical-signal-to-noise-ratio (OSNR) and absolute signal power. Because system performance is limited by the OSNR of the worst performing wavelength, a large variation can severely limit system length. The maximum difference of the gain within the spectral range of signals is called gain ripple. The gain ripple of an amplifier should be as small as possible. This can be achieved by properly selecting the pump wavelengths and powers of the Raman amplifier. As can be seen in FIG. 3, the gain ripple over the wavelength range of 1520 to 1620 nm is smaller than 1.5 dB.
FIG. 4 is a schematic of a typical optical communication system using Raman amplifiers for periodic amplification of the optical signal. The system includes transmitter terminal 10 and receiver terminal 12. The transmitter terminal includes a number of optical communication transmitters 14a, 14b, . . . 14z respectively transmitting signals at optical communications wavelengths λa, λb, . . . λz.
The optical signals are multiplexed by multiplexer 16 and are amplified by a series of amplifiers A1, A2, . . . An. The signals are transmitted from the transmitter 10 to the amplifiers, between the amplifiers, and from the amplifiers to the receiver 12 via transmission optical fiber 26. For distributed Raman amplification, the optical amplifier will also include transmission optical fiber. The optical signals are then demultiplexed by demultiplexer 18 of receiver 12 to respective optical communications receivers 20a, 20b, . . . 20z. The demultiplexer 18 sends optical communications wavelengths λa, λb, . . . λz to respective optical communications receivers 20a, 20b, . . . 20z. 
Although FIG. 4 shows signals directed from transmitter terminal 10 to receiver terminal 12 for ease of illustration, in general the transmitter terminal 10 and receiver terminal 12 are typically transmitter/receiver terminals for bidirectional communication. In this case each of the transmitter/receiver terminals will have transmitters as well as receivers and both a multiplexer and demultiplexer.
FIG. 5 is a schematic of a typical distributed Raman optical amplifier 50 employed as one of the amplifiers in the series of amplifiers A1, A2, . . . An in the system of FIG. 4. The amplifier 50 includes optical pump assembly 51 (shown enclosed by dashed lines) and transmission fiber 64. In this amplification scheme, the pump assembly 51 includes a pump radiation source 52 that provides, for example, twelve different pump wavelengths λ1 through λ12. Specifically, the pump radiation source 52 comprises twelve lasers 56 that each emit radiation at a different wavelength of the wavelengths λ1 through λ12. The radiation from the individual radiation sources 56 of the pump radiation source 52 are then coupled or combined at pump radiation combiner 54, and the coupled radiation is output at pump radiation combiner output 58.
The coupled radiation has a coupled radiation profile that is a combination of the individual radiation profiles of the radiation input into the pump radiation combiner 54. The pump radiation profile, that will be coupled with the optical signal to be amplified, is therefore the coupled radiation profile in this case. Thus, the pump radiation profile is output from output 58. The pump radiation profile from output 58 is then coupled at pump-signal combiner 60 with the optical signal 62. Optical signal 62, i.e., the data signal, propagates in the transmission optical fiber 64 in a direction opposite to the radiation of the pump radiation profile. The optical signal is amplified along transmission optical fiber 62. Thus, the amplifier 50 and pump assembly 51 provide amplification for a single optical transmission path.